The present invention relates to a roller which is suitable for the pressure treatment of webs of material, such as paper, and which has a device for equalizing the sag of the roller over its axial length.
Such a roller generally is used to define a roller press together with at least one counter roller. The roller press has a press nip between the two rollers through which the web of material to be treated is passed. The roller press can be used, e.g., as the dewatering press in a paper making machine or it can form a part of a calender serving for the smoothing of the paper. Numerous other possible uses for such a roller press can be envisioned.
Typically, such a roller comprises an annular, hollow shell or casing. As pressure is applied upon an elongated long roller, it tends to sag. With an annular roller shell, to counter this sag, the roller has a support passing through it. Various devices are known for supporting the shell of the roller against the support inside it to prevent the elongated shell from sagging. Through use of a device for equalizing sag, the pressing forces which act in the press nip may be set to be substantially the same over the entire axial length of the press nip or these pressing forces may be adjusted at different zones along the press nip to different values. The device for equalizing sag can be developed such that the roller shell can, if necessary, be lifted away from the counter roller and then be placed on the counter roller again.
In one roller with sag equalization, shown in U.S. Pat. No. 3,802,044, each of the supporting elements in the roller that serve to transmit pressing forces from the shell of the roller to the support inside the roller is developed as a hydraulic piston. The piston has a smooth surface at its end which rests against the inner surface of the roller shell. The piston is displaceable radially relative to the roller support. The piston together with the support inside the shell encloses a pressure space which is connected to a source of pressurized liquid. Since the roller support inside the shell sags relative to the roller shell upon the operation of the roller, a joint is provided between each piston and the support to allow for the relative motion of the roller support and the piston. In another known roller with sag equalization, shown in U.S. Pat. No. 3,846,883, the piston which is radially displaceable in the support cannot incline along with the support. Instead, a slide shoe is articulatedly connected with the piston between the inner surface of the roller shell and the piston.
In the two above noted roller constructions, measures are taken to reduce the frictional forces between the rotatable roller shell, on the one hand, and the nonrotating pistons or slide shoes, on the other hand. For this purpose, the slide surface of the piston or of the slide shoe, respectively, has hydrostatic pressure fields. These hydrostatic pressure fields are supplied with pressurized liquid by conduits leading from the pressure space present between the respective piston and the common roller support. One disadvantage of known rollers is that the pressurized liquid conduits must have throttling devices with very narrow flow cross-sections. If the pressurized liquid contains small particles of dirt, and this can never be entirely avoided, there is a danger that the throttle points in the liquid conduits will become clogged so that feeding of pressurizing liquid to the hydrostatic pressure fields will be reduced or interrupted. This may result in impermissibly high frictional forces occurring, upon operation of the roller, at the slide surfaces between the roller shell and the pistons or slide shoes. The slide surfaces could even be damaged. Furthermore, much more power would be necessary for driving the roller than when the hydrostatic pressure fields are operating properly. To avoid this problem, FIGS. 2 and 3 of U.S. Pat. No. 4,058,878 show a screen 30 that is provided to prevent clogging of the throttle channels 27.
In the known roller of U.S. Pat. No. 3,846,883, there is the additional disturbing factor that frictional forces also occur upon the relative movements between the slide shoe on the piston and the piston beneath the slide shoe. For this reason, another pressure relief space is provided between the piston and the slide shoe for relieving the pressure in the joint between the piston and the slide shoe. One disadvantage of this embodiment is its high cost.